Erythropoiesis dominates hematopoiesis during fetal life to accommodate the rapid expansion in fetal blood volume. Some developmental differences are known to distinguish fetal from adult erythropoiesis such as changes in globin expression and responsiveness to erythropoietin by erythroid progenitors. We hypothesize that a number of unknown genes are differentially regulated between fetal and adult erythroid precursors to accommodate the distinct functions of fetal and adult erythrocytes. Microarray technology is a powerful tool used to compare gene expression between cell populations. To best apply this technology to decipher gene expression during fetal and adult erythropoiesis we have embarked on an effort to improve the tools used to isolate erythroid precursors at different stages of maturation. A panel of single-chain variable fragment antibodies (scFv Abs) was developed, using phage display technology, which recognizes immature nucleated erythrocytes. We aim to determine the molecular identity and expression pattern of the cell-surface markers recognized by these novel reagents. At least three cell surface markers appear to be recognized by the scFv Abs with unique but overlapping expression during erythropoiesis. In addition, to identify the gene products recognized by the scFv Abs we propose to characterize the expression of these cell surface molecules during fetal and adult erythroid development. The second aim of this proposal is to profile gene expression at different stages of fetal and adult erythroid development. This will be accomplished by isolating erythroid precursors at different stages of differentiation from fetal liver and adult bone marrow and then analyzing gene expression using microarray technology. These experiments will both profile the expression of genes during erythroid development and compare expression between adult and fetal erythrocytes. A detailed understanding of gene expression that occurs during the growth and maturation of erythroid progenitors is likely to contribute to improved methods of genetic therapy for hemoglobinopathies. [unreadable] [unreadable]